The present invention relates generally to sensors, and more particularly, but not exclusively, to communication between a stationary component and one or more sensors located on a rotating component.
Maintaining an electrical connection between a stationary element and a rotating element often requires solutions that avoid static wires between the elements since, for example, a sensor located on a continuously rotating shaft would likely eventually tear out a conductive wire connecting the rotating sensor to a stationary signal processor located off the rotating shaft.
Slip rings are electromechanical devices that enable power and/or electrical signals to be transmitted between a rotating component and a stationary component. Slip rings are used in a variety of devices, including dissolution test instruments, radar antennas, gun turrets, periscopes, electro-optic sensor gimbals, and helicopter rotors. A conventional slip ring employs a stationary brush-like graphite or metal contact that rubs against the outside diameter of a corresponding rotating metal ring. The brush is electrically connected to the stationary component, and the corresponding metal ring is electrically connected to the rotating component. As the metal ring turns, the stationary brush provides a conductive path for electrical current to the metal ring to complete the connection between the stationary component and the rotating component. Problematically, these slip rings introduce electrical noise to the signal—which is detrimental to signal transmission. In addition, these slip rings typically need maintenance to avoid the degradation of the rotating electrical connection caused by debris and normal wear.
An improvement over the conventional brush-like slip ring is a mercury-filled slip ring coupled to a wired stationary element and a rotating shaft. An exemplary usage of a mercury-filled slip ring is described in U.S. Pat. No. 5,589,649 for a dissolution testing apparatus, which is incorporated by reference in its entirety herein. A mercury-filled slip ring uses a pool of liquid mercury instead of a brush to maintain electrical contact between a stationary component and a corresponding rotating component. Mercury-filled slip rings provide low resistance and stable connectivity, which, together, greatly reduce (i) electrical noise introduced into transmitted signals, as well as (ii) the ordinary debris and wear from brush slip rings. The low noise is particularly important for systems that transmit data signals across the slip ring.
However, mercury—in both its liquid and gaseous states—is toxic and, consequently, mercury-filled slip rings are not suitable for many applications, including food manufacturing and processing, pharmaceutical equipment, or other applications in which contamination poses a health risk due to potential mercury leakage. Additionally, since mercury solidifies at approximately −40° C., mercury slip rings are not suitable for use at lower temperatures.